Nanofabrication using focused ion beam and reactive ion etching techniques was conducted on single-crystal aluminium substrates with three crystallographic orientations: (111), (100), and (110). Among these, the (110) surface exhibited the highest etch rate of 15±2 nm/min and the lowest surface roughness of 2±0.5 nm using reactive ion etching. This enhanced understanding of how crystal orientation affects surface quality is expected to contribute significantly to the advancement of low-loss diffractive optical elements across the infrared to extreme ultraviolet spectral ranges. This is the first time that etching on single-crystal aluminium has been investigated with focused ion beam and reactive ion etching, providing references for potential optical device fabrication.